How to Choose a Gravity Separator for Grain and Seed

A gravity separator is usually the most expensive single machine in a small grain cleaning line, and the one buyers understand least. It does not sort by size. It sorts by density — how heavy a kernel is for its size.

That single fact explains both what it can do and what people expect from it wrongly.

What a gravity separator can and cannot do

It separates sound grain from kernels that are lighter than they should be: shrivelled, immature, sprouted, mould-affected, insect-damaged. It is the machine that turns "clean grain" into "graded grain", and for seed it is the machine that decides germination.

It cannot replace a destoner. Stones are heavier than grain and need their own machine. If a supplier tells you one machine will do both jobs properly, ask for a sample test.

Four things that decide the result

1. Deck area versus throughput

Capacity is not a number on a brochure — it depends on the material, the deck area and the cleanliness target. Wheat at 98% cleanliness is a different duty from vegetable seed at 99.5%. Always ask for capacity quoted for your material, and ask on what basis it was measured.

2. Air control

The air flow across the deck is what lifts light kernels. If it is a single fixed setting, the machine will work for one material only. Look for independent, adjustable air control per deck zone — this is what lets the same machine run wheat, corn, soybean and seed.

3. Adjustment on site

Deck angle, vibration amplitude, air volume and feed rate should all be adjustable, with marked positions you can return to. If adjusting the machine needs tools and a technician, it will never be adjusted between materials.

4. Single deck or double deck

A single-deck machine passes material once: light fraction out, sound grain out. A double-deck machine removes the light fraction on the upper deck and grades the sound grain on the lower deck. Double deck gives a cleaner separation and higher capacity in one footprint, at a higher price. For heavily contaminated material or a strict seed standard, it is usually the right answer.

The three-question test before you buy

1. Send a sample. A supplier who will not test your grain is selling a shape, not a result.
2. Ask what happens on a bad day. What if the incoming grain is wet, or mixed with another crop? A machine with adjustment range handles it; a machine sized to one ideal sample does not.
3. Ask about wear parts and access. Decks, screens and bearings need to be reachable. In a remote plant, standard parts matter more than a clever design.

What we need to quote one

Material and bulk density, current contamination, target cleanliness, tonnes per hour, and whether you run one crop or several. Tell us those and we will recommend single or double deck with a realistic capacity figure for your grain — including when the honest answer is that a smaller machine will do the job.

Send your sample details or browse the gravity separator range.